Optical scanning system with variable focus lens

ABSTRACT

Apparent distance of a pixel within an optical field of view is determined. Incoming light is scanned along a raster pattern to direct light for a select pixel onto a light distance detector. The distance is sampled for each pixel or for a group of pixels. The light distance detector includes a concentric set of rings sensors. The larger the spot of light corresponding to the pixel, the more rings are impinged. The diameter of the spot is proportional to the distance at which the light originated (e.g., light source or object from which light was reflected). Alternatively, a variable focus lens (VFL) adjusts focal length for a given pixel to achieve a standard spot size. The distance at which the light originated correlates to the focal length of the VFL.

CROSS REFERENCE TO RELATED APPLICATIONS

1. This is a continuation of U.S. patent application Ser. No. 09/188,991filed Nov. 9, 1998 for “Method and Apparatus for Determining OpticalDistance,” of Melville et al. The content of that application isincorporated herein by reference and made a part hereof.

BACKGROUND OF THE INVENTION

2. This invention relates to methods and apparatus for determining anoptical distance, such as a distance of an object within a field ofview, and more particularly to a method and apparatus for scanningdistances within a field of view.

3. A conventional camera includes an objective lens and a lightdetector, such as a photographic film, CCD array or other photosensitivedevice or structure. Light from a viewing environment enters the camerathrough the objective lens and impinges on the light detector. Theportion of the viewing environment for which light enters is thecamera's field of view. Some cameras pass the light to a viewfinder oreyepiece allowing an operator to select a desired field of view from thebackground environment. To take a picture or record, the light detectorcaptures frames of the background light from the field of view.

4. Often the field of view is divided into discrete picture elements orpixels. In conventional digital video cameras the light detector recordsdata for each pixel within the field of view for a given video frame.The data includes color, intensity and the pixel coordinates (i.e., x, ycoordinates).

5. Conventional still cameras and video cameras include optics forfocusing within the field of view. Thus, an operator can select to focuson a near field object or a far field object. Some cameras even includeautofocus devices which automatically adjust the focal length of theobjective lens to focus within the field of view.

SUMMARY OF THE INVENTION

6. According to the invention, an apparent distance of one or morepoints within an optical field of view is determined. For example, anapparent distance is determined for each pixel, or for one or more groupof pixels, within a field of view. Such distance is also referred to asa depth of view. One advantage of the invention is that pixel data foran object viewed may be recorded, input to a computer and mappedenabling display of a 3-dimensional model of the object. Anotheradvantage is that an augmented display device or camera device can havevariable accommodation.

7. Incoming light is scanned along a raster pattern to direct light fora select pixel onto a light distance detector. The distance is sampledfor each pixel or for a group of pixels. In one embodiment a lightdistance detector is used. In an alternative embodiment a variable focuslens is used.

8. The light distance detector includes a concentric set of ringsensors. The larger the spot of light corresponding to the pixel, themore rings are impinged. For light entering from a far distance, such asfrom infinity to about 20 feet, the spot will be small. For light comingfrom closer distances the spot is larger. The diameter of the spot isproportional to the distance at which the light originated (e.g., lightsource or object from which light was reflected). Each ring correspondsto a distance. The number of rings impinged determines the distance forthe pixel being sampled.

9. The variable focus lens (VFL) is included in the light path. For agiven pixel to be sampled, the focal length of the VFL is varied toachieve a small spot size. The distance at which the light originatedcorrelates to the resulting focal length of the VFL.

10. Although, distance is sampled for each pixel or for a group ofpixels, light intensity and color also may be sampled to record adigital image of a field of view, such as for a camera implementation.

11. The invention will be better understood by reference to thefollowing detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

12.FIG. 1 is a diagram of a conventional image detection apparatus;

13.FIG. 2 is a diagram of an apparatus for scanning optical distancewithin a field of view according to an embodiment of this invention;

14.FIG. 3 is a diagram of the light detector of FIG. 2;

15.FIG. 4 is a diagram of the light detector of FIG. 3 with an impingingspot of light;

16.FIG. 5 is a diagram of an electro-mechanically variable focus lensfor a lensing system of FIG. 2 according to an embodiment of thisinvention;

17.FIG. 6 is a diagram of an alternative variable focus lens embodimentfor the lensing system of FIG. 2;

18.FIG. 7 is a diagram of another alternative variable focus lensembodiment for the lensing system of FIG. 2;

19.FIG. 8 is a diagram of a plurality of cascaded lens for the lensingsystem of FIG. 2 according to an embodiment of this invention;

20.FIG. 9 is a block diagram of a feedback control scheme for detectinglight distance according to an embodiment of this invention; and

21.FIG. 10 is a diagram of an image recording apparatus according to anembodiment of this invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

22. Overview

23. Referring to FIG. 1, in a conventional image detection apparatus 10,background light from a field of view F impinges on an objective lens 14which converges the light toward a light detector 16. In a digitalcamera the light detector 16 may be a charge-coupled device (CCD), whichalso serves as a viewfinder. Light from objects within the field of viewF, such as a first object 18 (e.g., a tree) and a second object 20(e.g., a bird) is captured to record an image of the field of view or apart thereof.

24. Referring to FIG. 2, an apparatus 30 detects optical distance (i.e.,depth of view) for objects 18, 20 in the field of view F according to anembodiment of this invention. The apparatus 30 includes an objectivelens 14, a scanning system 32, a lensing system 34 and a light distancedetector 36. Background light 12, from the field of view F, includinglight reflected from the objects 18, 20 enters the apparatus 30 at theobjective lens 14. The light is directed to the scanning system whichscans the background light along two axes to select at any given time apixel area of the field of view to be analyzed. Light originating withinthe select pixel area is directed into the lensing system 34 whichconverges the light onto the light distance detector 36. Preferably onlylight originating from a single, select pixel area is focused onto thelight distance detector 36. In alternative embodiment the size of thearea being measured for distance may vary to include multiple pixels.The size of the field portion measured for distance is determined by thesize of a mirror surface on scanners 38, 40 within the scanner system32, the relative location of the mirror surface relative to theobjective lens 14 and the lensing system 34, and the relative locationof the lensing system 34 relative to the light distance detector 36.

25. During operation, the scanning system 32 periodically scans along aprescribed scanning pattern, such as a raster pattern. For scanning atwo dimensional raster pattern, a horizontal scanner 38 scans along ahorizontal axis and a vertical scanner 40 scans along a vertical axis. Asample is taken at the light distance detector for multiple points alongeach given horizontal scanning line. Such sample, for example,corresponds to a pixel. The light distance detector signal 35corresponds to the depth of view of the light sample. In someembodiments a table of correlation data is stored in memory 37. Acontroller 43 compares the light distance detector signal 35 to entriesin the table to derive the depth of view for the light sample. Thedetermined depth of view is read from the memory 37 and stored as thedepth of view for the pixel that was sampled. Thus, a distance (i.e.,depth of view) is determined for each pixel within the field of view.

26. In some embodiments the distance is stored in memory together withthe pixel coordinates (i.e., field of view coordinates) for laterretrieval. Light intensity and color also may be detected and stored, asfor a camera or other recording implementation.

27. Light Distance Detector

28. Referring to FIG. 3, a light distance detector 36 according to oneembodiment of this invention includes concentrically positioned lightdetection sensors 42-50 that form a set of concentric rings. The numberof rings and radial increment may vary depending on the distanceresolution desired. Light 52 from select pixel region is converged bythe lensing system 34 onto the light distance detector 36. Referring toFIG. 4, such light 52 forms a spot 54, preferably centered at the centerof the detector 36. The smaller the spot 54, the farther the focalsource of the light 52 for the select pixel. For light, at approximately20 feet or further from the system 30, the light waves are flat andfocus down to a common point size. Accordingly, light at such distanceis not differentiated (i.e., resolved). Light from zero feet toapproximately 20 feet from the system 30, however is differentiated byidentifying which ring sensors detect light. In the example illustratedin FIG. 4, the light spot 54 encompasses sensors 42-48. A specificdistance corresponds to activation of such sensors 42-48.

29. An alternative method for detecting the optical distance for pixellight is achieved, by modifying the focal length of the lensing system34 until a spot of a desired standard size is achieved. For example, thefocal length may be varied until the spot size encompasses only sensors42 and 44. Alternatively, only sensor 42 may define the standard spotsize or only sensors 42-46, or some other prescribed subset of sensors42-50 may define the prescribed spot size. Following is a description ofa lensing system which can vary its focal distance.

30. Lensing System with Variable Focal Length

31. To vary the focal length, the lensing system 14 includes a variablefocus lens (VFL). In some embodiments the VFL has its focus varied bycontrolling the shape or thickness of the lens. In other embodiment theVFL has its focus varied by varying the index of refraction of the lens.FIG. 5 shows an electro-mechanically variable focus lens (VFL) 60 whichchanges its shape. A central portion 62 of the VFL 60 is constructed ofa piezoelectric resonant crystalline quartz. In operation, a pair oftransparent conductive electrodes 64 provide an electrical field thatdeforms the piezoelectric material in a known manner. Such deformationchanges the thickness of the central portion 62 along its optical axisto effectively change the focus of the VFL 60. Because the VFL 60 is aresonant device, its focal length varies periodically in a verypredictable pattern. By controlling the time when a light pulse entersthe resonant lens, the effective focal position of the VFL 60 can becontrolled.

32. In some applications, it may be undesirable to selectively delaypulses of light according to the resonant frequency of the VFL 60. Insuch cases, the VFL 60 is designed to be nonresonant at the frequenciesof interest, yet fast enough to focus for each image pixel.

33. In an alternative embodiment, the variable focus lens is formed froma material that changes its index of refraction in response to anelectric field or other input. For example, the lens material may be anelectrooptic or acoustooptic material. In the preferred embodiment, thecentral portion 62 (see FIG. 11) is formed from lithium niobate, whichis both electrooptic and acoustooptic. The central portion 62 thusexhibits an index of refraction that depends upon an applied electricfield or acoustic energy. In operation, the electrodes 64 apply anelectric field to control the index of refraction of the lithium niobatecentral portion 62. In another embodiment a quartz lens includes atransparent indium tin oxide coating that forms the electrode 64.

34. In another embodiment shown in FIG. 6, a lens 70 includes acompressible cylindrical center 72 having a gradient index of refractionas a function of its radius. A cylindrical piezoelectric transducer 74forms an outer shell that surrounds the cylindrical center 72. When anelectric field is applied to the transducer 74, the transducer 74compresses the center 72. This compression deforms the center 72,thereby changing the gradient of the index of refraction. The changedgradient index changes the focal length of the center 72.

35. In another embodiment shown in FIG. 7 the variable focus element isa semiconductor device 80 that has an index of refraction that dependsupon the free carrier concentration in a transmissive region 82.Applying either a forward or reverse voltage to the device 80 through apair of electrodes 84 produces either a current that increases thefree-carrier concentration or a reverse bias that depletes the freecarrier concentration. Since the index of refraction depends upon thefree carrier concentration, the applied voltage can control the index ofrefraction. Memory 86 and control electronics 88 may be used to controlthe index of refraction.

36. In still another embodiment shown in FIG. 8 a plurality of lenses90-92 are cascaded in series. One or more piezoelectric positioners94-96 move one or more of the respective lenses 90-92 along the lightpath changing the focal distance of the light beam. By changing therelative position of the lenses to each other the curvature of the lightvaries.

37. According to one control approach, the lensing system 34continuously varies its focal length as needed to maintain a constantspot size. Referring to FIG. 9 the light distance detector 36 andlensing system 14 are coupled in a feedback loop. The output of thelight distance detector 36 is fed to focal control electronics 100. Thefocal control electronics 100 vary the focal length of a VFL 102 tomaintain a constant spot size (e.g., the prescribed standard spot sizepreviously described). The focal length at any given sample timecorrelates to the light distance (i.e., depth of view) for such sample.

38. According to another control approach, the lensing system performs asweep of the focal length range of the VFL during each light sample tobe measured. During the sweep the spot 54 (see FIG. 4) will achieve itssmallest size. The focal length at such time is used to define the lightdistance.

39. According to these control techniques, the precise light distancefor any given sample is determined from the focal length of the lensingsystem 14 at the time such sample is taken. One of ordinary skill in theart will appreciate that a specific distance can be derived from thefocal length using the various optical parameters (e.g., magnificationfactors, relative positions of components) of a system 30 embodiment.

40. Scanning System

41. In one embodiment, the scanning system 32 includes a resonantscanner for performing horizontal scanning and a galvanometer forperforming vertical scanning. The scanner serving as the horizontalscanner receives a drive signal having a horizontal scanning frequency.Similarly, the galvanometer serving as the vertical scanner receives adrive signal having a vertical scanning frequency. Preferably, thehorizontal scanner has a resonant frequency corresponding to thehorizontal scanning frequency. In other embodiments the vertical scanneralso is a resonant scanner.

42. One embodiment of a resonant scanner is described in related U.S.patent application (Attorney Docket No. OT2.P17 filed on the same dayand having the same inventive entity) Ser. No. 09/188,993 filed Nov. 9,1998 of Michael Tidwell et al. for Scanned Beam Display With AdjustableAccommodation. The content of that application is incorporated herein byreference and made a part hereof. The resonant scanner includes a mirrordriven by a drive circuit (e.g., electromagnetic drive circuit orpiezoelectric actuator) to oscillate at a high frequency about an axisof rotation. The drive circuit moves the mirror responsive to a drivesignal which defines the frequency of motion.

43. Referring to FIG. 2, background light 12 impinges on the mirror 39of one scanner 38, then is reflected to another scanner 40, where itsmirror 41 deflects the light toward the lensing system 34. As thescanner mirrors 39, 41 move, different portions (e.g., pixel areas) ofthe background field of view are directed toward the lensing system 34and light distance detector 36.

44. In alternative embodiments, the scanning system 32 instead includesacousto-optical deflectors, electro-optical deflectors, or rotatingpolygons to perform the horizontal and vertical light deflection. Insome embodiments, two of the same type of scanning device are used. Inother embodiments different types of scanning devices are used for thehorizontal scanner and the vertical scanner.

45. Image Capturing System

46. Referring to FIG. 10, an image capturing system 150 is shown inwhich image data is obtained and stored for each pixel within the fieldof view F for a single still frame or for multiple video image frames.The system 150 operates in the same manner as described for the system30 of FIG. 2 and like parts performing like functions are given the samepart numbers. In addition to detecting light distance however, lightintensity and light color also is detected for each pixel within thefield of view. Accordingly, a light intensity sensor 152 is includedalong with color sensor 154. One of ordinary skill in the art willappreciate that the sensors 152, 154 and 36 may be combined into acommon device, or that the color sensing and intensity sensing can beachieved with a common device. Further, rather than color detection grayscales may be detected for black and white monochromatic viewing.

47. For each pixel in the field of view, image data is obtained andstored in memory storage 156. The image data includes the pixelcoordinates, the determined light distance, the light intensity and thelight color. Such image data may be recalled and displayed at displaydevice 158 to replay the captured image frame(s). A controller 160coordinates the field of view scanning and the image replay.

48. Although preferred embodiments of the invention have beenillustrated and described, various alternatives, modifications andequivalents may be used. Therefore, the foregoing description should notbe taken as limiting the scope of the inventions which are defined bythe appended claims.

What is claimed is:
 1. A resonant scanning system, comprising: a firstlens receiving light from an optical field of view and directing thelight along a light path; a first surface receiving the directed lightfor redirecting at least a portion of the light; a second lens receivingthe redirected light and converging the redirected light; and a lightdetector which receives the converged light and generates a signalindicative of the depth of view for the sample area corresponding to theconverged light.
 2. The apparatus of claim 1 , wherein the first surfaceis a mirror which moves along a predetermined scanning path, wherein ata given time during the scanning path the mirror redirects light for aselect pixel within the field of view, the select pixel changing withtime during a scanning cycle, and wherein the light detector receivesconverged light for the select pixel, the select pixel corresponding tothe sample area.
 3. The apparatus of claim 1 , in which the lightdetector comprises a plurality of concentric light sensors, and whereinthe number of light sensors of the plurality of concentric light sensorswhich detect the converged light is indicative of the depth of view forthe sample area.
 4. The apparatus of claim 1 , wherein the second lensis a variable focus lens which receives a signal to cause a change infocal length of the variable focal lens, wherein the focal length of thevariable focus lens which results in a minimal spot size of convergedlight on the light detector corresponds to the depth of view of thesample area.
 5. The apparatus of claim 1 , wherein the second lens is avariable focus lens which receives a signal to cause a change in focallength of the variable focal lens, and further comprising a controllerreceiving the indicative signal, wherein the controller generates asignal for adjusting the focal length of the variable focus lens inresponse to the indicative signal to maintain the indicative signalconstant, and wherein the focal length of the variable focus lens at aselect sample time corresponding to the sample area is indicative of thedepth of view of the sample area.
 6. The apparatus of claim 1 , furthercomprising memory and wherein the light detector also indicates lightintensity and light color, wherein light intensity, light color, lightdepth of view and pixel coordinates are stored in memory for each one ofa plurality of pixels selected by the scanner.
 7. A variable focus lensapparatus in an optical scanning system, the apparatus comprising: alens with an alterable physical attribute; and means for dynamicallyaltering said physical attribute; wherein the altering means varies thefocus of the lens to allow the optical scanning system to measure anoptical distance of a sample of light.
 8. The apparatus of claim 7 ,wherein the physical attribute is one attribute from the group ofphysical attributes including: shape, thickness and index of refraction.9. The apparatus of claim 7 , in which the physical attribute is shape,said altering means comprising means for altering the shape of the lensto achieve a variation in focus of the lens.
 10. The apparatus of claim7 , in which the physical attribute is thickness, said altering meanscomprising means for altering the thickness of the lens to achieve avariation in focus of the lens.
 11. The apparatus of claim 7 , in whichthe physical attribute is index opf refraction, said altering meanscomprising means for altering the index of refraction of the lens toachieve a variation in focus of the lens.